The present invention relates to a novel free-cutting aluminum alloy which does not contain lead as an alloying element but only as a possible impurity. The invention further relates to processes for the production of such alloy and to the use thereof. The alloy exhibits superior strength properties, superior workability, superior free-cutting machinability, corrosion resistance, requires less energy consumption and is environmentally friendly in production and use. The present alloy is preferably intended to replace free-cutting alloys of the group AlCuMgPb (AA2030).
Free-cutting aluminum alloys were developed from standard heat treatable alloys, to which additional elements for forming softer phases in the matrix were added. These phases improve the machinability of the material during cutting by obtaining a smooth surface, while requiring decreased cutting forces and providing decreased tool wear. Chip breakage is also especially improved.
These softer phases are formed by alloying elements that are not soluble in aluminum, do not form intermetallic compounds with aluminum and have low melting points. Elements with these properties are lead, bismuth, tin, cadmium, indium and some others, which are not applicable for practical reasons. Said elements added individually or in combinations are precipitated during solidification in the form of globulite inclusions having a particle size from a few xcexcms to some tens of xcexcms.
The most important free-cutting aluminum alloys are:
Alxe2x80x94Cu with 0.2-0.6 wt. % Pb and 0.2-0.6 wt. % Bi (AA2011);
Alxe2x80x94Cuxe2x80x94Mg with 0.8-1.5 wt. % Pb and up to 0.2 wt. % Bi (AA2030); and
Alxe2x80x94Mgxe2x80x94Si with 0.4-0.7 wt. % Pb and 0.4-0.7 wt. % Bi (AA6262).
In these alloys, inclusions are formed for the purpose of easier machinability, especially through the use of lead and bismuth. Recently, there has been a tendency to replace lead with other elements because of risks to human health and for ecological reasons. As substitutes, tin and partly indium are most frequently used. The possibility of using tin in aluminum free-cutting alloys has been well-known for a long time. Tin was one of the first elements to be added to aluminum free-cutting alloys in amounts up to 2 wt. %. In practice, the use thereof, on a larger scale, has never taken place because of an alleged impairment of corrosion properties, poorer alloy ductility and high price. Recently, tin has been added, especially to alloys of the groups Alxe2x80x94Mgxe2x80x94Si (AA6xxx series) and Alxe2x80x94Cu (AA2xxx series) containingxe2x80x94when in standard formxe2x80x94lead and bismuth, or lead only.
Alloys with tin should have similar or better properties as to microstructure, workability, mechanical properties, corrosion resistance and machinability in comparison with standard alloys. The formation of suitable chips of alloys with tin dependsxe2x80x94similarly as in alloys with lead and bismuthxe2x80x94on the effect of inclusions for easier cutting upon the mechanism of breaking the material during cutting.
Earlier investigations and explanations of the mechanism of breaking chips have been based particularly on alloys containing lead and bismuth. Both elements form softer phases in a harder basis and retain their chemical and metallographic characteristics. At discontinuity sites, cohesion forces are weaker and, thus, the desirable breaking of chips during machining is facilitated. The distribution of globulite phases should be fine and uniform. A simultaneous addition of smaller amounts of two or more elements insoluble in aluminum has a greater effect upon machinability than the addition of one element. The elements are present in globulite phases in ratios equaling the analytical averages thereof.
It is known on the basis of practical experience that the breaking of chips is best at an eutectic composition of the elements insoluble in aluminum. Thus, the opinion prevails that a suitable breaking of chips is a result of the melting of said inclusions at temperatures attained during the working of the material by turning, boring, etc.
The present invention relates to novel free-cutting aluminum alloys that do not contain lead as an alloy element and further relates to processes for the production of these alloys and to the use thereof. The present alloy possesses superior strength properties, superior workability, superior machinability, corrosion resistance, requires less energy consumption and is environmentally friendly in production and use.
These improved properties and a lowering of the production costs are attained by means of an optimum selection of alloying elements, working processes and thermomechanical treatments.
The present invention provides a free-cutting aluminum alloy containing:
a) as alloy elements:
0.5 to 1.0 wt. % Mn,
0.4 to 1.8 wt. % Mg,
3.3 to 4.6 wt. % Cu,
0.4 to 1.9 wt. % Sn,
0 to 0.1 wt. % Cr,
0 to 0.2 wt. % Ti,
b) as impurities:
up to 0.8 wt. % Si,
up to 0.7 wt. % Fe,
up to 0.8 wt. % Zn,
up to 0.1 wt. % Pb,
up to 0.1 wt. % Bi,
up to 0.3 wt. % total of all remaining impurities, and
c) the balance substantially 100% aluminum.
The alloy containing 1.1 to 1.5 wt. % Sn is preferable.
The alloy containing up to 0.06 wt. % Pb is preferable.
The alloy containing up to 0.05 wt. % Bi is preferable.
The invention further provides a process for working and thermal treatment of the above alloy by semicontinuous casting, homogenization annealing, cooling from the homogenization annealing temperature, heating to the working temperature of extrusion, comprising novel and inventive process measures of carrying out an indirect extrusion at the maximum temperature of 380xc2x0 C., press-quenching and natural aging.
According to a variant of the above process, the indirect extrusion step is conducted at a maximum temperature of 380xc2x0 C., press-quenching and artificial aging are conducted at a temperature of from 130 to 190xc2x0 C. for 8 to 12 hours.
According to a further variant of the above process, the indirect extrusion is conducted at a maximum temperature of 380xc2x0 C., followed by press-quenching, cold working and natural aging.
According to a further variant of the above process, the indirect extrusion is conducted at a maximum temperature of 380xc2x0 C., followed by press-quenching, cold working and artificial aging at a temperature from 130 to 190xc2x0 C. for 8 to 12 hours.
According to a further variant of the above process, the indirect extrusion is conducted at a maximum temperature of 380xc2x0 C., followed by press-quenching, tension straightening and natural aging.
According to a further variant of the above process, the indirect extrusion step is conducted at a maximum temperature of 380xc2x0 C., followed by press-quenching, tension straightening and artificial aging at a temperature from 130 to 190xc2x0 C. for 8 to 12 hours.
According to a further variant of the above process, the indirect extrusion step is conducted at a maximum temperature of 380xc2x0 C., followed by press-quenching, cold working, tension straightening and natural aging.
According to a further variant of the above process, the indirect extrusion is conducted at the maximum temperature of 380xc2x0 C., followed by press-quenching, cold working, tension straightening and artificial aging are conducted at a temperature from 130 to 190xc2x0 C. for 8 to 12 hours.
A further object of the invention is a product obtained according to the above process or variants thereof, having a tensile strength of 293 to 487 N/mm2, a yield stress of 211 to 464 N/mm2, a hardness HB of 73 to 138 and an elongation at failure of 4.5 to 13%.
A further object of the invention is a product obtained according to the above process or variants thereof, having a tensile strength of 291 to 532 N/mm2, a yield stress of 230 to 520 N/mm2, a hardness HB of 73 to 141 and an elongation at failure of 5.5 to 11.5%.
Alloys made according to the present invention are divided into five groups with respect to their tin content.
1st group: 0.40 wt. % Sn to 0.70 wt. % Sn
2nd group: 0.71 wt. % Sn to 1.00 wt. % Sn
3rd group: 1.01 wt. % Sn to 1.30 wt. % Sn
4th group: 1.31 wt. % Sn to 1.60 wt. % Sn
5th group: 1.61 wt. % Sn to 1.90 wt. % Sn
Alloys have to be divided with respect to their tin content because an increasing tin content at a constant content of other alloying elements and impurities causes a reduction of strength properties after thermal treatment. On the other hand, an increasing tin content results in the formation of more favorable chips during machining.
At a constant content of alloying elements and impurities and under the same conditions of casting, homogenization annealing, working with extrusion and thermal treatment, the mechanical properties and machinability of semi-finished products from alloys depend upon the tin content. An increasing tin content improves machinability with respect to an easier chip breaking. A higher tin content results in smaller chips. An increasing tin content causes a lower tensile strength and yield stress.
Cutting conditions affect the machinability of alloys containing tin. At higher cutting rates with tools made of carbide hard metal alloys, also at lower tin contents ( less than 1.2 wt. % Sn), favorable chips are obtained.
Alloys with lower tin contents have poorer chips at lower cutting rates and good chips at higher cutting rates. Alloys with lower tin contents have higher mechanical properties in comparison with alloys having higher tin contents.
Alloys with higher tin contents have favorable chips at all cutting rates. Alloys with higher tin contents have lower mechanical properties in comparison with alloys with lower tin contents.
The tin content limit affecting the obtaining of favorable or unfavorable chips as well as higher or lower mechanical properties is 1.2 wt. % Sn.
The invention comprises novel processes for the working and thermal treatment of the above aluminum alloys with tin. Semi-finished products made of standard free-cutting alloys of the group AlCuMgPb in the form of rods having a circular or hexagonal cross section are usually manufactured according to the following processes:
Process 1 (T3).
Semicontinuous casting, homogenization annealing, cooling from the homogenization annealing temperature, heating to the working temperature of extrusion, extrusion, solution annealing (usually in a salt bath for alloys of the group AA2xxx), quenching, cold deformation with drawing, natural aging.
Process 2 (T4).
Semicontinuous casting, homogenization annealing, cooling from the homogenization annealing temperature, heating to the working temperature of extrusion, extrusion, solution annealing (usually in a salt bath for alloys of the group AA2xxx), quenching, natural aging.
Process 3 (T6).
Semicontinuous casting, homogenization annealing, cooling from the homogenization annealing temperature, heating to the working temperature of extrusion, extrusion, solution annealing (usually in a salt bath for alloys of the group AA2xxx), quenching, artificial aging.
Process 4 (T8).
Semicontinuous casting, homogenization annealing, cooling from the homogenization annealing temperature, heating to the working temperature of extrusion, extrusion, solution annealing (usually in a salt bath for alloys of the group AA2xxx), quenching, cold deformation with drawing, artificial aging.
Novel processes for the manufacture, working and thermomechanical treatment of the inventive alloy of the group AlCuMg with Sn relate to (1) a change of working temperatures, which are higher than in conventional processes, (2) introduction of indirect extrusion with higher extrusion rates, (3) press-quenching directly after the extruded piece exits the die, (4) increased degrees of cold deformation during thermomechanical treatment, (5) optimum temperatures and time periods of artificial aging, and (6) processes for achieving a stress-free state in extruded and thermomechanically treated rods.
The introduction of novel processes for working and thermomechanical treatment of alloys is advantageous over conventional processes for the following reasons:
By various combinations of technological processes after the extrusion of the alloy, it is possible to achieve various controlled mechanical properties of semi-finished products and technological properties such as improved machinability and surface quality.
The inventive technological processes for working and thermomechanical treatment show the following advantages in comparison with semi-finished products made using standard alloys of the group AlCuMgPb according to the conventional processes:
Quicker extrusion of the material in the indirect extrusion press.
By press-quenching, the working heat is utilized for solution annealing. According to this process, separate solution annealing, usually taking place in salt baths, may be omitted. Thus, less energy and working times are required. It will also be appreciated that in this way, ecological problems in connection with the use of a salt for solution annealing are also solved. (Alloys of the group AA2xxx, in which the conventional alloy AlCuMgPb (AA2030) belongs, are prepared according to a process of separate solution annealing.)
Due to the use of press-quenching, the alloys have a smooth and light surface. In conventional processes with separate solution annealing, a darker surface is formed because of the oxidation of magnesium on the rod surface, the effect of salt corrosion. Mechanical damage to the extruded rod surfaces caused by manipulating in several handling operations required in conventional processing is eliminated by the process of the present invention.
By combining cold deformation and the degree of the cold deformation before natural or artificial aging, strength properties increased. Mechanical properties (yield stress, tensile strength) of the inventive alloys with tin are lower than those of the conventional alloy AlCuMgPb (AA2030).
By combining cold deformation before natural or artificial aging, internal stresses are minimized.
By introducing deformation before the aging of extruded rods, a stress-free state in semi-finished products is achieved.
The invention also comprises the following processes in the manufacture and thermal treatment of the novel alloy with tin:
Process a.
Process a. comprises the following steps:
Semicontinuous casting of bars; homogenization annealing of the semicontinuously cast bars for eight hours at 490xc2x0 C.; cooling the bars after homogenization to ambient temperature with a cooling rate of 230xc2x0 C./h; heating the bars to a working temperature of 380xc2x0 C.; and indirect extrusion of the bars or billets into rods with diameters from 12 mm to 127 mm followed by quenching of the extruded rods. The invention also comprises cooling the extrusion toolxe2x80x94the diexe2x80x94with liquid nitrogen. The die must be cooled because of the high working temperatures necessary for a successful solution annealing at the extrusion press. The quenching of the extruded pieces after leaving the die takes place in a water wave. The maximum permissible time between the working and the quenching of the material is 30 seconds. The maximum permissible cooling of the surface of the extruded pieces before quenching is 10xc2x0 C. Natural aging of the quenched, extruded pieces takes six days.
Process b.
Process b. comprises the following steps:
Semicontinuous casting of bars; homogenization annealing of the semicontinuously cast bars for eight hours at 490xc2x0 C.; cooling the bars after homogenization to ambient temperature with a cooling rate of 230xc2x0 C./h; heating the bars to a working temperature of 380xc2x0 C.; and indirectly extruding the bars or billets into rods with diameters from 12 mm to 127 mm. The invention also comprises cooling the extrusion toolxe2x80x94the diexe2x80x94with liquid nitrogen. The die must be cooled because of the high working temperatures necessary for a successful solution annealing at the extrusion press. The quenching of extruded pieces after leaving the die takes place in a water wave. The maximum permissible time between the working and the quenching of the material is 30 seconds. The maximum permissible cooling of the surface of the extruded pieces before quenching is 10xc2x0 C. Artificial aging is conducted for 8 to 12 hours within a temperature range from 130 to 190xc2x0 C.
Process c.
Process c. comprises the following steps:
Semicontinuous casting of bars; homogenization annealing of the semicontinuously cast bars for eight hours at 490xc2x0 C.; cooling the bars after homogenization to ambient temperature with a cooling rate of 230xc2x0 C./h; heating the bars to a working temperature of 380xc2x0 C.; and indirectly extruding the bars or billets into rods with diameters from 12 mm to 127 mm. The invention also comprises cooling the extrusion toolxe2x80x94the diexe2x80x94with liquid nitrogen. The die must be cooled because of the high working temperatures necessary for a successful solution annealing at the extrusion press. The quenching of extruded pieces after leaving the die takes place in a water wave. The maximum permissible time between the working and the quenching of the material is 30 seconds. The maximum permissible cooling of the surface of the extruded pieces before quenching is 10xc2x0 C. Extruded and quenched rods are then drawn with a deformation rate of up to 15%. Natural aging of the drawn rods takes six days.
Process d.
Process d. comprises the following steps:
Semicontinuous casting of bars; homogenization annealing of the semicontinuously cast bars for eight hours at 490xc2x0 C.; cooling the bars after homogenization to ambient temperature with a cooling rate of 230xc2x0 C./h; heating the bars to a working temperature of 380xc2x0 C.; and indirectly extruding the bars or billets into rods with diameters from 12 mm to 127 mm. The invention also comprises cooling the extrusion toolxe2x80x94the diexe2x80x94with liquid nitrogen. The die must be cooled because of the high working temperatures necessary for a successful solution annealing at the extrusion press. The quenching of extruded pieces after leaving the die takes place in a water wave. The maximum permissible time between the working and the quenching of the material is 30 seconds. The maximum permissible cooling of the surface of the extruded pieces before quenching is 10xc2x0 C. Process d. also includes drawing the extruded and quenched rods with a deformation rate of up to 15%. Artificial aging for 8 to 12 hours is conducted within a temperature range from 130 to 190xc2x0 C. The final technological phase is a process for obtaining a stress-free state of semi-finished products in the form of rods.
The present novel alloys may also be thermally and thermomechanically treated according to processes of separate solution annealing, which correspond to processes according to the classification of Aluminum Association T3, T4, T6 and T8 (these processes marked by e, f, g and h in Table 1 are not objects of the present invention).
Process i.
Process i. comprises the following steps:
Semicontinuous casting of bars; homogenization annealing of semicontinuously cast bars for eight hours at 490xc2x0 C.; cooling the bars after homogenization to ambient temperature with a cooling rate of 230xc2x0 C./h; heating the bars to a working temperature of 380xc2x0 C.; and indirectly extruding the bars or billets into rods with diameters from 12 mm to 127 mm. The invention also comprises the cooling of the extrusion toolxe2x80x94the diexe2x80x94with liquid nitrogen. The die must be cooled because of the high working temperatures necessary for a successful solution annealing at the extrusion press. The quenching of extruded pieces after leaving the die takes place in a water wave. The maximum permissible time between the working and the quenching of the material is 30 seconds. The maximum permissible cooling of the surface of the extruded pieces before quenching is 10xc2x0 C. Process i. further includes tension straightening of extruded pieces in order to obtain a stress-free state followed by natural aging for six days.
Process j.
Process j. comprises the following steps:
Semicontinuous casting of bars; homogenization annealing of the semicontinuously cast bars for eight hours at 490xc2x0 C.; cooling the bars after homogenization to ambient temperature; heating the bars to a working temperature of 380xc2x0 C.; and indirectly extruding the bars or billets into rods with diameters from 12 mm to 127 mm. The invention also comprises the cooling of the extrusion toolxe2x80x94the diexe2x80x94with liquid nitrogen. The die must be cooled because of the high working temperatures necessary for a successful solution annealing at the extrusion press. The quenching of extruded pieces after leaving the die takes place in a water wave. The maximum permissible time between the working and the quenching of the material is 30 seconds. The maximum permissible cooling of the surface of the extruded pieces before quenching is 10xc2x0 C. Process j. also include tension straightening of the extruded pieces in order to obtain a stress-free state followed by artificial aging for 8 to 12 hours in a temperature range from 130 to 190xc2x0 C.
Process k.
Process k. comprises the following steps:
Semicontinuous casting of bars; homogenization annealing of the semicontinuously cast bars for eight hours at 490xc2x0 C.; cooling the bars after homogenization to ambient temperature with a cooling rate of 230xc2x0 C./h; heating the bars to a working temperature of 380xc2x0 C.; and indirectly extruding the bars or billets into rods with diameters from 12 mm to 127 mm. The invention also comprises the cooling of the extrusion toolxe2x80x94the diexe2x80x94with liquid nitrogen. The die must be cooled because of the high working temperatures necessary for a successful solution annealing at the extrusion press. The quenching of extruded pieces after leaving the die takes place in a water wave. The maximum permissible time between the working and the quenching of the material is 30 seconds. The maximum permissible cooling of the surface of the extruded pieces before quenching is 10xc2x0 C. Extruded and quenched rods are drawn according to Process k. with a deformation rate of up to 15% followed by tension straightening of the extruded pieces in order to obtain a stress-free state, followed by natural aging for six days.
Process l.
Process l. comprises the following steps:
Semicontinuous casting of bars; homogenization annealing of the semicontinuously cast bars for eight hours at 490xc2x0 C.; cooling the bars after homogenization to ambient temperature; heating the bars to a working temperature of 380xc2x0 C.; and indirectly extruding the bars or billets into rods with diameters from 12 mm to 127 mm. The invention also comprises the cooling of the extrusion toolxe2x80x94the diexe2x80x94with liquid nitrogen. The die must be cooled because of the high working temperatures necessary for a successful solution annealing at the extrusion press. The quenching of extruded pieces after leaving the die takes place in a water wave. The maximum permissible time between the working and the quenching of the material is 30 seconds. The maximum permissible cooling of the surface of the extruded pieces before quenching is 10xc2x0 C. Extruded and quenched rods are drawn according to Process l. with a deformation rate of up to 15%, followed by tension straightening of the extruded pieces in order to obtain a stress-free state, followed by artificial aging for 8 to 12 hours in a temperature range from 130 to 190xc2x0 C.